Gold nanomaterial has unique size-dependent optical, thermal, electrical, magnetic, and chemical properties. This feature is responsible for enhanced phenomena such as surface plasma resonance absorption (SPR), Raman scattering (RS), catalytic activity and biocompatibility. These properties enable gold nanomaterial to have important applications in catalysis, biosensors, and biomedical fields. Numerous research studies have shown that, the surface morphology, size and surface composition of the gold nanomaterial have a strong influence to the functional property of the gold nanomaterial. Therefore, the research of gold nanomaterial's synthesis method and its properties are important to both fundamental research and practical application of nanomaterials.
Functionalization of gold nanomaterial mainly includes two aspects: One is to achieve controllable morphology and size of gold nanoparticle for desirable SPR absorption properties and catalytic activity of the gold nanoparticle; the other aspect is to modify the surface of an existing gold nanomaterial, in order to enrich the surface of gold nanomaterial with different types of ligands to enable various activities, such as luminescence, redox reaction, biological activity and electrochemical reaction.
Study of synthesizing methods of spherical gold nanoparticle is the earliest research activity in the field of synthesizing gold nanomaterials with various morphological features. Currently, spherical gold nanoparticles can be readily prepared with various diameters in the range of several nanometers to hundreds of nanometers. These spherical gold nanoparticles can be dispersed very well. Although there is a frequency shift in SPR absorption spectrum corresponding to the sizes of the spherical gold nanoparticle, the range of this shift is not significant. Wherese a more effective means to regulate their SPR absorption features is providing different morphology of gold nanoparticle. It has been an attractive research subject in recent years to utilize anisotropic shape of gold nanomaterial, such as rod-like, or linear gold nanoparticle, gold nano sheets and gold nano flowers to provide unique adjustable SPR absorption properties.
Commonly, gold nanoparticles are protected by inorganic ion layer and organic molecule layer. The ion and molecule layers maintain their own optical, electrochemical or redox activity while stabilizing the gold nanoparticle. Therefore it is another effective means of obtaining the functionalized gold nano materials by modifying the surface of the nanoparticle with various kinds of organic molecules or biological molecule, which have a variety of optical, electrochemical activity, or doping these molecules to the interior of the nanomaterial. Due to the relatively dense internal structure of a metal nanoparticle, it is more common to chemically modify the surface of the metal nanoparticle to functionalize the nanomaterial. Utilization of the strong interaction between gold and sulfur and using mercaptan as the surface ligand to stabilize the gold nanoparticle is the most effective method of preparing stable gold nanomaterial in the recent decade (Brust, M.; thiol molecules Wallker, M.; Bethell, D J Chem. Soc. Chem. Commun., 1994, 801).
It has been one of the focus topics in nanoscience research that the special optical properties of the nanoparticle contain a wealth of information about the energy level structure and surface state of the nanoparticle Current studies of the optical properties of nanomaterials include mainly optical behaviors such as surface plasma resonance absorption, surface-enhanced Raman scattering and photoluminescence.
Chemiluminescence is a light emission phenomenon caused by a chemical reaction. It differs from the above optical phenomenon in that no external light source excitation is necessary for chemiluminescence. Advantages of chemiluminescence include high sensitivity, wide linear range, low background noise, and it is obtainable from simple, cheap instruments. In recent years, the application of metal nanomaterial in chemiluminescence has become an important research topic. Although chemiluminescence from semiconductor quantum dots and chemiluminescence induced directly or indirectly by metallic nano materials have been studied, current research work is still focused on chemiluminescence from unmodified nanomaterial directly, or light emitting from an unmodified nanomaterial induced under some other conditions. Research work regarding chemiluminescence from modified or functionalized gold materials has not been reported in the fields of theoretical analysis or applications of chemiluminescence. In the year 2005, Roux et al (Roux, S.; Garcia, B.; Bridot, J L; Salom, Marquette, C. Langmuir. 2005, 21, 2526) using dihydrolipoic acid as a protective agent together with sodium borohydride (NaBH4) as a reducing agent reduced chloroauric acid (chloroauric acid.3H2O) and synthesized gold nanoparticle under the protection of dihydrolipoic acid. In their method, the carboxyl group of dihydrolipoic acid on the surface of gold nanoparticle was activated by utilizing 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) and N-hydroxysuccinimide (NHS), then luminol was grafted onto the surface of the gold nanoparticle by condensation reaction between luminol-NH2 group and dihydrolipoic acid carboxyl-COOH group. This process is troublesome and time consuming, requires multi-steps such as precipitating, washing, vacuum distillation, filtration, dispersing, etc., to complete the reactions. The research results show that luminol bonded through dihydrolipoic acid bonding to the surface of the gold nanoparticle still has relatively good electrically excited chemiluminescence activity. In 2007, our group directly used luminol reducing chloroauric acid and synthesized luminol direct-bonded gold nanomaterial in one step (Cui, H.; Wang, W.; of Duan, C. R; Dong, Y P; Guo, J Z Chem. Eur. J. 2007, 13, 6975). Characteristic results show luminol is connected directly to the surface of the gold nanoparticle with a weak Au—N bonding. The luminol direct bonding gold nanoparticle were assembled onto the surface of the gold electrode through electrostatic interaction by cysteine bridging molecules and we found that the modified electrodes had chemiluminescence activity (ECL) under electrical stimulation in alkaline solution. The intensity of the chemiluminescence activity increased with the increase of concentration of H2O2, and thus we developed a H2O2 ECL sensor.
N-(4-aminobutyl)-N-ethylisoluminol is a luminol isomer, which is one of the derivatives from isoluminol. Its chemical formula is C14H20N4O2. It is a white powder at room temperature, and is a stable synthetic organic compound. Its chemiluminescent characteristics is superior than that of luminol. Since the aliphatic amine group of N-(4-aminobutyl)-N-ethylisoluminol can be easily connecting to a marker, while maintains a high luminous efficiency, N-(4-aminobutyl)-N-ethylisoluminol is suitable for making a direct probe for biological analysis. Therefore, the functionalized gold nanomaterial synthesized by using N-(4-aminobutyl)-N-ethylisoluminol as a reducing agent directly to reduce chloroauric acid may achieve a higher chemiluminescence efficiency than that from the gold nanomaterial synthesized by the method of direct-bonding luminol to the gold nanomaterial. However, it was found that a stable N-(4-aminobutyl)-N-ethylisoluminol functionalized gold nanomaterial colloid cannot be obtained by replacing luminol with N-(4-aminobutyl)-N-ethylisoluminol under the condition of reducing chloroauric acid and synthesizing luminol bond to gold nanomaterial directly (i.e. Placing 100 mL of 0.01% (w/w) of HAuCl4 solution and heating it to the boiling point. Under the condition of thoroughly stirred in a reflux condenser, adding 1.5-2.0 mL of 0.01 mol/L luminol and 0.01 mol/L sodium hydroxide solution while maintaining boiling and continuously stirring the reflux for 30 min. then removing the heating source and continuously stirring for 15 min to get the gold nanomaterial solution). Therefore, it is important to explore the synthesizing method of N-(4-aminobutyl)-N-ethylisoluminol functionalized gold nanomaterial, its chemiluminescence characteristics and its bioanalytical applications.